UltraCAD
Design, Inc.
ESR and Bypass Capacitor Self Resonant Behavior
How to Select Bypass Caps
Douglas G. Brooks, MS/PhD
Rev 2/21/00
Introduction
Self Resonant Frequencies
It’s no news that the band pass requirements for power systems on PCB’s are increasing and that power supply impedance requirements are getting tighter. Bypass capacitor fabrication and assembly techniques are improving and pushing
higher the normal self-resonant frequencies we have to deal
with. ESR’s (equivalent series resistance) are decreasing,
pushing further down the floor of the power supply impedance
curve.
Assume a simple capacitor with capacitance C, inductance L, and equivalent series resistance (ESR) equal to
R. The inductance should be considered from the practical sense — i.e. not only the inherent inductance associated with the capacitor physical structure itself, but also
the PCB pads and attachment process, etc. The impedance through this capacitor is:
All this has created increased debate as to how to take advantage of this higher self-resonant requency and lower ESR. One
argument is that lower ESR is thoroughly beneficial. Another
is that, while lower ESR lowers the impedance at the minimum points, it also increases it at the maximum (“antiresonant”) points, and therefore lower ESR is not necessarily
beneficial. Some argue for system designs that incorporate a
well defined number of high quality (precise self-resonant frequencies and low ESR) capacitors with carefully chosen selfresonant frequencies. Others argue for more general quality
bypass capacitors with SRF’s (self-resonant frequencies) well
spread across the frequency range of interest.
Z = R + jwL + 1/jwC, or
Z = R + j(wL - 1/wC)
where w is the angular frequency:
w = 2 * Pi * f
Resonance occurs, by definition, when the j term is zero:
wL = 1/wC
w2 = 1/LC
w = 1/Sqrt(LC)
The impedance through the capacitor at resonance is R.
And here is a point to ponder. In the past, with large numbers
of capacitors spread all over our boards, “anti-resonant” peaks
have not generally been regarded as an issue. How did we get
away with that for so long?
Here are some “truths” that can (and will) be demonstrated in
this paper:
1.
2.
3.
4.
As ESR goes down, the troughs get deeper and the peaks
get higher.
The minimum impedance value is not necessarily ESR
(or ESR/n, where n is the number of identical parallel capacitors); it can be lower than that!
The impedance minimums are not necessarily at the self
resonant points of the bypass capacitors.
For a given number of capacitors, better results can be
obtained from more capacitor values, with moderate
ESRs, spread over a range than with with a smaller set of
capacitor values, with very low ESRs, at even wellchosen specific self resonant frequencies.
Effects of multiple capacitors
Assume we have n identical caps, as above. The equivalent circuit of the n identical capacitors is the single capacitor whose values are
C = nC
L = L/n
R = R/n
The impedance of this system is now
Z = R/n + j( wL/n - 1/wnC)
The resonant frequency of this system is, again, where
the j term goes to zero, or where
wL/n = 1/wnC
UltraCAD Design, Inc. 11502 NE 20th, Bellevue, WA. 98004 Phone: (425) 450-9708 Fax: (425) 450-9790
ultra@ultracad.com
ftp.ultracad.com
http://www.ultracad.com
Copyright 2000 by UltraCAD Design, Inc.
Figure 1.
Difference in frequency response between a single capacitor and n
parallel capacitors.
which results in exactly the same self-resonant frequency
as before. Paralleling capacitors does not change the selfresonant frequency, but it effectively increases the capacitance, reduces the inductance, and reduces the ESR compared to a single capacitor. The resulting impedance response curve tends to “flatten out” compared to a single
capacitor, see Figure 1.
Historically, on circuit boards, circuit designers have used
a large number of bypass capacitors of “the same” value
(the reason for the quotes will become evident later!). The
advantage of this process has been the increased C and the
reduced L and R that results.
Parallel Capacitors
C1 > C2
L1 > L2
R2
L1
L2
C1
C2
Figure 2.
Two capacitors in parallel
The combined impedance through the system is:
Z=
1
1
1
+
Z1 Z 2
(R + jX 1)(R + jX 2 )
=
2 R + j ( X 1 + X 2)
From this, we can derive the real and imaginary terms of
the impedance expression:
Re( Z ) =
Take the case of two parallel capacitors, shown in Figure
2. Let’s let R1 = R2 = R in order to simplify the arithmetic.
(This assumption does little harm and greatly helps the
intuition!) Let us also assume that:
R1
Im(Z ) =
[(
)
R 2 R 2 − X 1X 2 + ( X 1 + X 2 )
2
4 R 2 + ( X 1 + X 2)
]
( X 1 + X 2)(R 2 + X 1X 2)
2
4R 2 + ( X 1 + X 2)
Further, we derive that the magnitude and phase of the
impedance term are:
which means that Fr1 (the self-resonant frequency of C1)
is lower than Fr2. Now:
X1 = wL1 - 1/wC1
Z1 = R + jX1
2
X2 = wL2 - 1/wC2
Z2 = R + jX2
Z = Re( Z ) 2 + Im( Z ) 2
 Im( Z ) 

Θ = Tan −1 
 Re( Z ) 
Figure 3
Impedance curve for two capacitors in parallel
The curve of impedance as a function of frequency is
shown in Figure 3. It is instructive to look at this curve,
and the real and imaginary terms of the impedance expression formula together.
Let Im(Z) Equal Zero
Resonance occurs when the imaginary term is zero. This
is also the point at which the phase angle is zero. The impedance at that point is simply the real part of the impedance expression.
The imaginary term for Z goes to zero under two conditions:
X1 = -X2
R2 = -X1X2
The first condition would represent the “pole” between
the self-resonant frequencies of the two capacitors if R
were zero. Since R > 0, there is not a “true” pole for any
real value of frequency. But X1 equals –X2 when the reactance term of C1 is inductive (+) and increasing, the
reactance term for C2 is capacitive (-) and decreasing,
and where the two reactance terms are equal. This is the
“anti-resonance” point that occurs at a frequency between
Fr1 and Fr2.
Assuming R is small, the second condition can only occur
where either X1 or X2 is small. X1 is small near Fr1 and
X2 is small near Fr2. X1 and X2 must be of opposite
sign, since R2 must be positive. Therefore, these resonant
points must be between Fr1 ad Fr2, and they must not be
equal to Fr1 or Fr2 (unless, in the limit, R = 0).
The system resonant frequencies are not necessarily the
same as the capacitor self resonant frequencies unless
ESR is zero.
It can further be shown that at this point, where the imaginary term is zero and R2 = -X1X2, the real term, and thus
the impedance itself, simply reduces to R.
Impedance at Fr1
At Fr1, the self-resonant frequency of C1, X1 = 0. It can be
shown that:
Θ =Tan-1(RX2/(2R2+X22)
If X1 = 0, then X2 must be negative (capacitive, under the
conditions we have been assuming) so
Θ<0
Only in the limit where R = 0 does Θ go to zero.
The magnitude of impedance at the point where X1 = 0 can
be shown to be:
Z =R
R 2 + X 22
4R 2 + X 22
This
is less than R for any
value of R > 0. In the limit, it is equal to R for R = 0 and
equal to R/2 if R>>X2.
The results are exactly symmetrical if we are looking at Fr2,
the point where X2 = 0.
The minimum value for the impedance function is at a frequency other than the self resonant frequency of the capacitor and less than ESR when two capacitors are connected in
parallel. Further, the minimum value declines as X2 gets
smaller, or, as the self resonant frequencies of the capacitors are moved closer together, or, as the number of capacitors increases. This point is illustrated in Appendix 3.
Impedance at “Anti-resonance”
If we let X1 = -X2, then Im(Z) goes to zero, by definition.
This is the “anti-resonant” point between Fr1 and Fr2. At
this point, it can be shown that:
Z=
R X2
+
2 2R
For small values of R, this is inversely proportional to R
and can be a very large number if R << 0. This is why
there is concern about very high impedances at the “antiresonant” point. If R, on the other hand, is only in the
range of .1 or .01, then this number might be more manageable.
But consider this. If Z equals (approximately) R at the
minimum, under what conditions is Z also equal to R at
the maximum? Under those conditions, the impedance
curve will be (at least approximately) flat! It turns out that
Z equals R if:
R = X1 = -X2
We can achieve a (relatively) flat impedance response
curve if we position our capacitor values such that, at the
“anti-resonant” points, X1 = -X2 = ESR.
This has a very significant consequence. As ESR gets
smaller, then, for a flat impedance response, X1 and X2
must be smaller at the anti-resonant points. This means
that Fr1 and Fr2 must be closer together. And THIS means,
that as ESR gets smaller, it requires more capacitors to
achieve a relatively flat impedance response! This point is
highlighted graphically in Appendix 4.
General Case Analysis
As we add more values for C, the algebra associated with
these kinds of analyses gets very difficult. We at UltraCAD wrote our own program so we could look at various
capacitor configurations and see what happens in a more
“real world” situation.
The program is both elegant and inelegant at the same
time! It is elegant in that it actually works, works easily,
and it gets to an answer! It is inelegant in that it reaches an
answer by “brute force” calculations that can take a fair
amount of time in a complex case. And, it does not solve
for exact maximum and minimum impedance values (and
frequencies) but gets only arbitrarily close (but as you will
see below, close enough).
The program operates in two modes, (1) internally selected
capacitor values and (2) user supplied values. Using the
first mode, there must be at least two capacitor values, .1
uF and .001 uF. Inductance associated with these two values are 10 nH and .1 nH, respectively. If additional capacitors are used, their capacitive and inductive values are
spread logarithmically over this range. The user enters
ESR separately, which is assumed constant for all values
of capacitance. The specific program code looks like this:
‘ user has entered nvalues, number of capacitor values
‘ user has entered nsame, number of caps of same value
For i = 1 To nvalues
C(i) = (0.1 * ((0.01) ^ ((i - 1) / (nvalues - 1)))) * 10 ^ (-6)
L(i) = (10 * ((0.01) ^ ((i - 1) / (nvalues - 1)))) * 10 ^ (-9)
Next i
For i = 1 To nvalues
Ctotal = Ctotal + C(i) * nsame
Next I
Note: Although this approach might, in fact, lead to an optimal
distribution of capacitance values, this technique was not chosen
for that purpose, and that property is not claimed for this distribution. The computer needed some rule for selecting capacitor
values; thus was simply the rule chosen.
Appendix 1 shows the first set of results. Three capacitor
values were chosen, .1, .01, and .001 uF. Ten capacitors of
each value were assumed. The inductance and the selfresonant frequency associated with each capacitor value
are shown in the individual tables. The conditions under
the three analyses shown in Appendix 1 were identical except that ESR is different for each case, being 0.00001,
0.001, and .1 Ohms, respectively.
The top portion of each output gives the general input conditions; the middle portion gives the calculated capacitance
and related inductance value, and the self-resonant frequency for each capacitor. The bottom portion of each table provides the results. It provides each (approximate)
turning point frequency in the impedance curve, whether
that turning point is a minimum or maximum point, and
the value of the impedance function at that point. It also
provides the phase angle of the impedance function at that
point.
For very low values of R, the phase angle changes very
rapidly as it passes through zero (which it does near (but
not necessarily exactly at) each turning point.)
Note from the results how dramatically the maximum and
minimum values of impedance depend on R. Also, note
how, when R is small, the minimum point actually begins
shifting outside the self resonant point of some capacitors.
The results from Appendix 1 are shown in graphical form
in the appendix.
Appendix 1 illustrates 30 capacitors, 10 each for three values of capacitance. What if, instead, we selected 30 individual capacitors spread evenly across the same range?
Appendix 2 illustrates the results, and it tabulates them for
approximately half the frequencies —– because of the way
the capacitor values are selected, the results are symmetrical for the higher frequencies in the table.
The results are dramatically better in Appendix 2 than in
Appendix 1 (middle table) for the same number of capacitors (30) and same ESR! The peaks and valleys are 40.2
and 0.0001 Ohms, respectively for 10 each of 3 values,
and only 1.0 and 0.001 Ohms, respectively, for 1 each of
30 values! This suggests that very acceptable results can
be achieved with:
1. a smaller number of capacitors
2. spread across a range of values, with
3. a nominal, but not exceedingly low ESR.
For the same number of capacitors and value for ESR,
best results are obtained by spreading the capacitance values across a range rather than groups of capacitors
around a given value.
This may explain why we have not had many problems in
the past. Historically, we have used bypass capacitor values with wide tolerances, therefore spread broadly across a
range, and with only moderate ESR values, just what this
analysis suggests is optimal.
Achieving a Smooth Response
As suggested above, we can achieve an (approximately)
flat frequency response if we place the self resonant frequencies of the capacitors close enough so that the following relationship applies at the anti-resonant frequency:
R = X1 = -X2
Appendix 3 illustrates what happens as we continue to increase the number of capacitors to what we sometimes see
on our boards. Capacitor values are selected so that the
self-resonant frequencies are optimally spaced between 5
MHz and 500 MHz. Three cases are shown, 100 capacitors, 150 capacitors and 200 capacitors, all with ESRs
of .01.
Of particular interest is that, for each case, the highest impedance values are lower than the lowest impedance values for the case before, at every frequency! This demonstrates that the minimum impedance is, indeed, below
ESR, and that as the capacitor values become closer together, the peaks drop dramatically.
1,67,4,.01
1,1,1.1,.001
20,.01,.9,.001
1,.0009,.00005,.00001
Input file for calculator mode2 operation. Data is for:
1 ea 67 uF caps with 4 nH inductance and .01 ESR
1 ea 1.0 uF caps with 1.1 nH inductance and .001 ESR
20 ea .01 uF caps with .9 nH inductance and .001 ESR
The fourth line simulates a plane with .0009 uF capacitance.
Figure 4
Input file illustration
Further, note that 200 capacitors with ESR of .01 and with
self-resonant frequencies placed optimally between 5 MHz
and 500 MHz provide a virtually flat impedance response
curve at 5 milliohms or less! Even the case with 150 capacitors results in a very flat impedance response curve.
Appendix 4 shows what happens when we use the same 150
capacitors as shown in Appendix 3, but lower their ESR
to .001 Ohms. The results are dramatically worse! This confirms what was stated above, that as ESR declines, it takes
more capacitors to achieve a given response function!
User Supplied Input Values
In operating mode 2, the user may enter up to 500 sets of
capacitor data. Each set of data (one record) consists of four
items of information (fields). The information, in this order,
includes:
The number of capacitors with these parameters
Capacitance, in uF
Inductance, in nH
ESR, in Ohms
Records do not have to have unique values for capacitance.
In fact, records need not even be unique.
Figure 4 illustrates a sample input file. It contains three records reflecting a total of 22 capacitors and one additional
record simulating the capacitance of a plane.
The output result from this input is shown in Appendix 5.
Note in particular the sharp impedance peak caused by the
anti-resonance between the bypass capacitors and the plane
capacitance.
Bypass Capacitor Impedance Calculator
The calculator used in this analysis is available from UltraCAD’s web site:
http://www.ultracad.com
The shareware version is limited to up to 3 each of up to 3 different capacitor values. It works in both modes described above. A license for the full function calculator is available for $75.00. Details and a mini-user’s manual are
available on the web site.
UltraCAD’s Bypass Capacitor Impedance Calculator
Appendix 1
Effects of Varying ESR
These three graphs correspond to the three
(output) cases tabulated on the next page.
They each model the case of:
3 capacitor values,
chosen internally by the program, with
10 caps of each value.
The difference between them is that is that the
ESR assumed for the caps varies. The assumed ESRs are:
Top:
Mid:
Bot:
.00001 Ohms
.001 Ohms
.1
Ohm
Note how lower ESR reduces the peaks and
tends to “flatten” the curves somewhat.
Appendix 1 (Cont.)
Effects of Varying ESR
Initial Conditions
R (Ohms)
Number of Capacitor Values
Number of caps for EACH Value
Total Capacitance
=
=
=
=
0.00001
3
10
1.11
uF
L nH
10.00000
01.00000
00.10000
C uF
.100000
.010000
.001000
Frequency (MHz)
5.0329
15.3599
50.3292
164.9127
503.292
Impedance
.0000010
4003.5583008
.0000010
3936.4686108
.0000010
Initial Conditions
R (Ohms)
Number of Capacitor Values
Number of caps for EACH Value
Total Capacitance
=
=
=
=
0.001
3
10
1.11
L nH
10.00000
01.00000
00.10000
C uF
.100000
.010000
.001000
Frequency (MHz)
5.0329
15.36
50.329
164.91
503.29
Impedance
.0001000
40.1688765
.0001000
40.1659130
.0001000
Initial Conditions
R (Ohms)
Number of Capacitor Values
Number of caps for EACH Value
Total Capacitance
=
=
=
=
0.1
3
10
1.11
L nH
10.00000
01.00000
00.10000
C uF
.100000
.010000
.001000
Frequency (MHz)
5.0059
15.368
50.329
164.82
506.01
Impedance
.0099777
.4066792
.0099797
.4066792
.0099777
R
0.00001
0.00001
0.00001
Turn
Min
Max
Min
Max
Min
Resonant F (MHz)
5.033
50.329
503.292
PhaseAngle(Rad)
-.8716
-4.7111
-1.5911
-11.501
-.9432
uF
R
0.001
0.001
0.001
Turn
Min
Max
Min
Max
Min
Resonant F (MHz)
5.033
50.329
503.292
PhaseAngle(Rad)
-.1728
-.6469
-.1527
.9541
-.1326
uF
R
0.1
0.1
0.1
Turn
Min
Max
Min
Max
Min
Resonant F (MHz)
5.033
50.329
503.292
PhaseAngle(Rad)
-3.9414
-1.1535
-.0015
1.1724
3.9422
These results come from three runs using identical values for the capacitors except for their ESR. There are 10 capacitors of
each value used in the analysis. The values for the capacitors are shown in the middle portion of each report. The bottom portion of the reports shows the minimum and maximum impedance values, the frequency (MHz) associated with that value, and
the phase angle (in degrees) of the impedance expression at that frequency. The minimum and maximum frequency points are
accurate to about .01%.
Appendix 2
Effects of Number of Capacitor Values
The black curve shows the impedance response from 10 each of three values for a total of 30 capacitors and 1.11
uF total capacitance. The red curve shows the results from the same number of capacitors (30), but with one each
spread over the same range of values. Although the total capacitance is less (only .67 uF), the overall response is
better. The output corresponding to the red curve is partially shown on the next page.
Appendix 2 (Cont.)
Effects of Number of Capacitor Values
Initial Conditions
R (Ohms)
Number of Capacitor Values
Number of caps for EACH Value
Total Capacitance
L nH
10.00000
08.53168
07.27895
06.21017
05.29832
04.52035
03.85662
03.29034
02.80722
02.39503
02.04336
01.74333
01.48735
01.26896
01.08264
00.92367
=
=
=
=
0.001
30
1
0.6752
uF
C uF
.100000
.085317
.072790
.062102
.052983
.045204
.038566
.032903
.028072
.023950
.020434
.017433
.014874
.012690
.010826
.009237
R
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
Resonant F (MHz)
5.033
5.899
6.914
8.104
9.499
11.134
13.05
15.296
17.929
21.014
24.631
28.87
33.838
39.662
46.488
54.488
<clip>
Frequency (MHz)
5.0327
5.2563
5.8989
6.2134
6.9142
7.3224
8.1042
8.6172
9.4989
10.132
11.134
11.907
13.05
13.986
15.296
16.423
17.928
19.28
21.014
22.629
24.631
26.557
28.87
31.162
33.838
36.563
39.662
42.898
46.488
50.329
54.488
Impedance
.0009989
.6347305
.0009993
.7897249
.0009995
.8781140
.0009996
.9346362
.0009996
.9724241
.0009997
.9987498
.0009997
1.0171292
.0009997
1.0300747
.0009998
1.0392816
.0009997
1.0456834
.0009998
1.0503213
.0009998
1.0534425
.0009997
1.0554540
.0009997
1.0565857
.0009997
1.0569449
.0009997
<clip>
Turn
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
PhaseAngle(Rad)
-3.3536
-3.6028
-2.4119
-2.5201
-1.6806
-1.76
-1.2704
-1.3348
-1.3302
.0143
-.1913
-1.4751
-.6241
-.4517
-.3987
-.2126
-1.3008
-.3874
-.3006
.4032
.3959
-.5864
.3725
-.0349
-.2245
.0406
.1786
-.019
.2741
.1568
-.1518
Appendix 3
Achieving a Smooth Response
These curves show the impedance response from a number of capacitors optimally placed with self-resonant frequencies between 5 MHz and 500 MHz. In the center region, the impedance range is approximately
100 Capacitors: .007 to .012 Ohms
150 Capacitors: .005 to .006 Ohms
200 Capacitors: .0046
Note that each successive curve is below the prior curve at every frequency.
Note: The apparent “banding” or modulation pattern in the graph for 100 capacitors is caused by the interaction of the
graphical program resolution and the screen resolution of the monitor from which this picture is taken.
Appendix 4
Another Illustration of the Impact of ESR
The red (second, or center, or gray) graph is the same data as the 150 capacitor model in Appendix 3. That was 150 capacitors, each with an ESR of .01. The larger, black graph shows the impedance curve with the same 150 capacitors, but
each with an ESR of .001. The average impedance is (roughly) the same, but the impedance curve for the lower ESR capacitors is higher than the other curve for over half the frequencies in the range!
Note: As before, the apparent pattern in the graph for ESR = .001 is caused by the interaction of the graphical program
resolution and the screen resolution of the monitor from which this picture is taken.
Appendix 5
General Case File Input Example
1,67,4,.01
1,1,1.1,.001
20,.01,.9,.001
1,.0009,.00005,.00001
Input File
Output File
Initial Conditions
Input filename
= C:\A4_in.txt
Output filename
= C:\A4_Out.txt
Number of Capacitance Values = 4
Total Capacitance
= 68.2009
Number
1
1
20
1
L nH
04.00000
01.10000
00.90000
00.00005
Frequency (MHz)
1.
2.1112
4.7989
12.518
53.052
812.38
C uF
67.000000
1.000000
.010000
.000900
Impedance
.0300925
.2574331
.0009993
3.3499338
.0000500
817.2332967
Turn
Min
Max
Min
Max
Min
Max
R
.01
.001
.001
.00001
PhaseAngle(Deg)
60.77
-8.0969
.2803
-.9026
.2375
1.7471
Resonant F (MHz)
.307
4.799
53.052
23725.418